Not sure if it helps, but I generally handled this by send a notification to myself, in plain text. Alternatively, you could collect the error message into a variable and then log the variable somewhere you can read.

Good Morning! I am not sure about the SharePoint online version. The architecture is completely different.

On the On on-premise version, you can look into the SP Logs of the WFE (Web Front End) servers. I believe the workflow message are in 'Workflow Infrastructure' category. Just search by 'Workflow In' in the logs message, you should be able to find relative error messages.